the cat is weak or has the wrong cat.
the cat must be lit off.
lit off happens 2 ways.
cat must be close to the exhaust header, and the ecu sends it extra wasted fuel to do that, (that is why 96 has tad lower mpg than 1995 this waste)
sure exhaust leaks of any kind will kill the 02, (im sure non of this is your problem)
just leaks.
outside air is 20% oxygen.
if you get even 1% of that , in to there, the 02 slams lean.
420 are harmless. its just a code to say the cat is lazy, at the least or missing. or dead. nothing more.
all exhausts systems go positive and negative pressure, at each and every put.
when negative,it will suck in air.
both ends off the cat must be air leak free,
this is a real pressure plot . no leaks allowed,
![[Image: exhaustscavenging-p1-1.JPG]](http://www.fixkick.com/exhaust/exhaustscavenging-p1-1.JPG)
front leaks, cause EFI to waste huge fuel.
rear leaks cause fibbing 420s. just one tiny crack and bam 420s and is a lie,. the new cat works only the test fails.
the cat must be lit off.
lit off happens 2 ways.
cat must be close to the exhaust header, and the ecu sends it extra wasted fuel to do that, (that is why 96 has tad lower mpg than 1995 this waste)
sure exhaust leaks of any kind will kill the 02, (im sure non of this is your problem)
just leaks.
outside air is 20% oxygen.
if you get even 1% of that , in to there, the 02 slams lean.
420 are harmless. its just a code to say the cat is lazy, at the least or missing. or dead. nothing more.
all exhausts systems go positive and negative pressure, at each and every put.
when negative,it will suck in air.
both ends off the cat must be air leak free,
this is a real pressure plot . no leaks allowed,
front leaks, cause EFI to waste huge fuel.
rear leaks cause fibbing 420s. just one tiny crack and bam 420s and is a lie,. the new cat works only the test fails.
http://www.fixkick.com